Semiconductor device and production method thereof

ABSTRACT

On a silicon substrate 1 is provided a silicon oxide film 2, on which a polycrystalline silicon film 3 is formed by a low pressure CVD method at a monosilane partial pressure of no more than 10 Pa and a film formation temperature of no lower than 600° C. The polycrystalline silicon film is doped with an impurity such as phosphorus in a concentration of 1×10 20  atoms/cm 3  to 1×10 21  atoms/cm 3  to form a phosphosilicate glass film 6, and after removing it, the polycrystalline silicon film is thermally oxidized in an oxidative atmosphere to form a dielectric film 5 on the surface. A polycrystalline silicon film 4 is formed on the dielectric film 5, which is treated as the oriented polycrystalline silicon film 3a to form an oriented polycrystalline silicon film 4a. The oriented polycrystalline silicon film 4a as an upper electrode and the oriented polycrystalline silicon film 3a as a lower electrode are wired to obtain a semiconductor device having a capacitor. Further, a thin film transistor of a high dielectric strength can be produced in a short time on the polycrystalline silicon which is oriented in a short time.

This application is based on U.S. patent application Ser. No. 07-139,633filed Jun. 6, 1995 in Japan, the content of which is incorporatedfereinto by reference. In addition, this application is a continuationapplication of International Application No. PCT/JP96/01541 filed Jun.6, 1996.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device having adielectric film formed by thermal oxidation of polycrystalline siliconand to a production method thereof. More specifically, the presentinvention relates to a semiconductor device having a dielectric filmformed by providing polycrystalline silicon of improved film quality,followed by thermal oxidation and to a production method thereof.

DESCRIPTION OF RELATED ART

A capacitor as a component of IC and LSI is an indispensable part for ICand LSI of a specific field. In general a capacitor of IC and LSI has astructure as shown in FIG. 1 in which a dielectric film 5 is sandwichedbetween polycrystalline silicon films 3 and 4 on a silicon oxide film 2on a silicon substrate 1. The polycrystalline silicon films 3 and 4 areindividually a lower electrode and an upper electrode, which areindividually connected with lead wires 7.

In the production method of such a prior art capacitor, first thepolycrystalline silicon film 3 is formed on the silicon oxide film 2formed on the substrate 1 by way of low pressure CVD (chemical vapordeposition) at a pressure of 28 Pa (monosilane partial pressure: 14 Pa).The polycrystalline silicon film 3, to enhance the electroconductivityas a capacitor electrode, is doped with an impurity of a dopingconcentration of about 1×10¹⁹ atoms/cm³. In this case, the crystalorientation of the polycrystalline silicon film includes (220), (311),and the like in addition to (111). Next, the dielectric film 4 as anactive part of the capacitor is formed by thermal oxidation of thepolycrystalline film so as to have a film thickness and an areaaccording to the required capacitance. Then, the polycrystalline siliconfilm 4 is formed as in the polycrystalline silicon film 3.

A capacitor is normally required to have a dielectric strength of about8 MV/cm for the dielectric film in order to maintain the reliability.However, there may be a rare case that an abnormal overvoltage ismomentarily applied in addition to the rated voltage, or the dielectricfilm is contaminated with, for example, a metal, or defects of the filmoccur in the production process of the dielectric film of the capacitor,resulting in degradation of the dielectric strength of the dielectricfilm.

Further, as a capacitor, there is a problem in that leak current is highbetween the upper and lower electrodes.

Then, the thickness of the dielectric film has heretofore been increasedto enhance the dielectric strength of the dielectric film itself and toreduce the leak current.

However, a high capacitance cannot be obtained by the method ofincreasing the dielectric film thickness as used in the prior art.Therefore, in order to obtain a high capacitance, it is necessary toincrease the surface area, which results in an increased size of thesemiconductor device. Further, with increasing down-sizing requirementsfor semiconductor devices, if a thin film structure of the dielectricfilm is used to reduce the capacitor size, it is difficult to maintainthe dielectric strength and suppress an increase in leak current.

OBJECT OF THE INVENTION

An object of the present invention is to provide a semiconductor deviceand production method thereof which solve the above described prior artproblems.

Another object of the present invention is to provide a semiconductordevice having a capacitive component that can maintain the dielectricstrength without increasing the thickness of the dielectric film andsuppress an increase in leak current and a production method thereof.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the present invention, whichattains the above object, there is provided a semiconductor devicecomprising a polycrystalline silicon layer whose main crystalorientation is oriented in (111), an SiO₂ layer contacting thepolycrystalline silicon layer obtained from the polycrystalline siliconlayer, and an electrode contacting the SiO₂, wherein a height differenceof surface irregularities of the polycrystalline silicon or thedielectric film is no greater than 30 nm.

Here, the dielectric film preferably has a dielectric strength of noless than 8 MV/cm.

According to a second aspect of the present invention, there is provideda capacitor having a dielectric film between an upper electrode and alower electrode, wherein the lower electrode is a polycrystallinesilicon layer having a high conductivity whose main crystal orientationis oriented in (111), and the dielectric film is an SiO₂ layer obtainedfrom the polycrystalline silicon layer.

The polycrystalline silicon layer preferably contains an impurity in aconcentration of 1×10²⁰ atoms/cm³ to 1×10²¹ atoms/cm³.

Further, the polycrystalline silicon layer or the dielectric filmpreferably has a height difference of surface irregularities of lessthan 30 nm.

Still further, the dielectric film preferably has a dielectric strengthof no less than 8 MV/cm.

According to a third aspect of the present invention, there is provideda production method of a semiconductor device having a polycrystallinesilicon layer and an SiO₂ layer obtained from the polycrystallinesilicon layer, comprising the steps of: forming the polycrystallinesilicon layer by a low pressure CVD method at a monosilane partialpressure of no more than 10 Pa and a film formation temperature of nolower than 600° C.; heat treating the thus formed polycrystallinesilicon layer for doping it with an impurity and orienting its maincrystal orientation in (111); and thermally oxidizing the surface of theoriented polycrystalline silicon layer to form an SiO₂ film.

In this case, prior to thermally oxidizing the surface of the orientedpolycrystalline silicon layer, it is preferable to remove thehigh-concentration oxide film formed on the surface of thepolycrystalline silicon layer.

According to a fourth aspect of the present invention, there is provideda production method of a capacitor having a dielectric film between alower electrode and an upper electrode, comprising the steps of: forminga polycrystalline silicon layer as the lower electrode by a low pressureCVD method at a monosilane partial pressure of no more than 10 Pa and afilm formation temperature of no lower than 600° C.; heat treating theformed polycrystalline silicon layer for doping with an impurity andorienting its main crystal orientation in (111); and thermally oxidizingthe surface of the oriented polycrystalline silicon layer to form anSiO₂ film.

In this case, the polycrystalline silicon layer may be formed on thedielectric film by the low pressure CVD method at a monosilane partialpressure of no more than 10 Pa and a film formation temperature of nolower than 600° C., and then heat treatment may be made for doping withan impurity and orienting its main crystal orientation in (111).

Further, it is preferable to make doping with the impurity to aconcentration of 1×10²⁰ atoms/cm³ to 1×10²¹ atoms/cm³.

Still further, prior to thermally oxidizing the surface of the orientedpolycrystalline silicon layer, it is preferable to remove thehigh-concentration oxide film formed on the surface of thepolycrystalline silicon layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing the sectional structure ofa prior art capacitor;

FIG. 2 is a schematic sectional view showing the sectional structure ofthe capacitor according to the present invention;

FIG. 3 is a schematic sectional view showing the sectional structure ofthe thin film capacitor according to the present invention;

FIG. 4 is a schematic view showing the shapes of a polycrystallinesilicon film as a lower electrode and of a dielectric film of thecapacitor according to the present invention;

FIG. 5 is a schematic sectional view showing the shapes of apolycrystalline silicon film as a lower electrode and of a dielectricfilm of a capacitor of Comparative Example 2;

FIG. 6 is a schematic sectional view showing the shapes of apolycrystalline silicon film as a lower electrode and of a dielectricfilm of a prior art capacitor;

FIG. 7A to FIG. 7E are schematic sectional views illustrating theproduction process of the capacitor according to the present invention;

FIG. 7A is a schematic sectional view showing the structure in a stagewhen a silicon oxide film and a polycrystalline silicon film are formedon a substrate;

FIG. 7B is a schematic sectional view showing the structure in a stagewhen a phosphosilicate glass film is formed on the polycrystallinesilicon film and the polycrystalline silicon is oriented;

FIG. 7C is a schematic sectional view showing the structure in a stagewhen the phosphosilicate glass film is removed;

FIG. 7D is a schematic sectional view showing the structure in a stagewhen the oriented polycrystalline silicon film is thermally oxidized toform a dielectric film;

FIG. 7E is a schematic sectional view showing the structure in a stagewhen a similarly oriented polycrystalline silicon film is formed on thedielectric film;

FIG. 8A to FIG. 8E are schematic sectional views illustrating theproduction process of the thin film transistor according to the presentinvention;

FIG. 8A is a schematic sectional view showing the structure in a stagewhen a silicon oxide film and a polycrystalline silicon film are formedon a substrate;

FIG. 8B is a schematic sectional view showing the structure in a stagewhen a phosphosilicate glass film is formed on the polycrystallinesilicon film and the polycrystalline silicon is oriented;

FIG. 8C is a schematic sectional view showing the structure in a stagewhen the phosphosilicate glass film is removed;

FIG. 8D is a schematic sectional view showing the structure in a stagewhen the oriented polycrystalline silicon film is thermally oxidized toform a dielectric film;

FIG. 8E is a schematic sectional view showing the structure in a stagewhen a similarly oriented polycrystalline silicon film is formed on thedielectric film.

BEST MODE FOR PRACTICING THE INVENTION

Herein, the term main crystal orientation indicates a directionperpendicular to the main orientation plane, wherein the mainorientation plane is defined as an orientation plane which gives thegreatest ratio among those normalized as a ratio of the strength oforientation plane to the total strength of a sample in the analysis byXRD or the like.

The present invention will now be described in detail with reference tothe accompanying drawings.

FIG. 2 shows the sectional structure of the capacitor according to thepresent invention. As shown in FIG. 2, a silicon oxide film 2 isprovided on a silicon substrate 1, and a polycrystalline silicon layer3a is formed on the silicon oxide film 2. The polycrystalline siliconlayer 3a is highly electroconductive, and the main orientation thereofis in (111). On the polycrystalline silicon layer 3a is formed adielectric film 5 comprising SiO₂ layer, which is a thermal oxide of thepolycrystalline silicon. On the dielectric film 5 is formed apolycrystalline silicon layer 4a, which is highly conductive similar tothe polycrystalline silicon layer 3a and whose main crystal orientationis oriented in (111). That is, the polycrystalline silicon layers 3a and4a sandwich the dielectric film 5 therebetween, with the polycrystallinesilicon layers 3a and 4a being a lower electrode and an upper electrode,respectively. Further, these electrodes are individually connected withrespective lead wires 7.

FIG. 3 is a schematic sectional view showing the sectional structure ofa thin film transistor having the dielectric film structure according tothe present invention. In FIG. 3, there are formed on, for example, ann-type silicon substrate 10, a source portion 12S of the reverseconductive type (for example, p-type) to the silicon substrate, a gateportion 12G of the same conductive type (for example, n-type) as thesilicon substrate, and a drain portion 12D of the reverse conductivetype (for example, p-type) to the silicon substrate, through a siliconoxide film 11. The source portion 12S, the gate portion 12G, and thedrain portion 12D comprise polycrystalline silicon layers whose maincrystal orientations are oriented in (111). The source portion 12S andthe drain portion 12D are highly conductive. The gate portion 12G isdoped with an impurity in such a concentration that the thresholdvoltage of the thin film transistor is not too high for practical use. Agate dielectric film 13 is provided so that the gate portion is coveredtherewith. The gate dielectric film 13 comprises SiO₂, which is athermal oxide of the polycrystalline silicon layer 12a and whose maincrystal orientation is oriented in (111), forming the source portion12S, the gate portion 12G, and the drain portion 12D. In the thin filmtransistor according to the present invention, a height difference ofsurface irregularities of the polycrystalline silicon layer or thedielectric film is no greater than 30 nm. The source portion 12S, thedrain portion 12D, and a gate electrode 14a are individually connectedwith respective lead wires 12.

The semiconductor device having the above-described layered structurecan be produced by a conventional method known in the art, for example,by a low pressure CVD method. However, formation of the SiO₂ layer as athermal oxide of the polycrystalline silicon layer whose main crystalorientation is oriented in (111) is carried out according to the presentinvention as follows. Specifically, when polycrystalline silicon isformed on the silicon oxide film on the silicon substrate by the lowpressure CVD method, partial pressure of monosilane is set to no higherthan 10 Pa, thereby forming polycrystalline silicon in a state that isreadily oriented in (111) by subsequent heat treatment. The crystal isoriented in (111) by heat treatment in which the polycrystalline siliconfilm is doped with an impurity as heat treatment after the filmformation. In the case of the gate dielectric film of the thin filmtransistor, the concentration of the impurity for doping thepolycrystalline silicon is set to a high concentration so that thethreshold voltage is within the practically tolerable range.

As the dopant, phosphorus is typically used, and arsenic or boron can beused as well. The orientation is accelerated when the dopantconcentration is high compared to low concentration. The treatmenttemperature for doping with phosphorus is normally 800° C. to 1,000° C.,preferably 950° C. The temperature when the oriented polycrystallinesilicon film is heat-treated to form a dielectric film is normally 950°C. to 1,150° C., preferably 1,000° C. The polycrystalline silicon filmwhose main crystal orientation is oriented in (111) has a regularcolumnar structure, and the surface state is also regular even with someirregularities in height. The state is shown in FIGS. 4, 5, and 6. FIG.4 is a schematic sectional view showing the sectional structures of thepolycrystalline silicon film and of the dielectric film in the stage ofstep 4 in Example 1 of the present invention, FIG. 5 is a schematic viewcorresponding to FIG. 4 when the doping phosphorus concentration is lowin step 2 in Example 1, and FIG. 6 is a schematic view corresponding toFIG. 2 in the prior art production method. FIGS. 4 to 6 are schematicviews illustrating the results obtained by a transmission electronmicroscope. Since the surface irregularities are relatively large inamount in FIG. 5, it can be seen that the surface state is more improvedin high-concentration doping (FIG. 4). In the state obtained by theprior art method as shown in FIG. 6, the shape of polycrystallinesilicon is irregular with large surface steps, having large amounts ofsteep changes. The height difference of surface irregularities is about20 nm to 25 nm in the case of FIG. 4, whereas 35 nm to 50 nm in the caseof FIG. 6.

In the state of FIG. 4 according to the present invention, thedielectric film obtained by thermally oxidizing the surface of thepolycrystalline silicon film has no steep step portions and thus doesnot tend to cause electrical field concentration. Further, the filmformation speed by thermal oxidation depends on the crystal orientationof polycrystalline silicon. Therefore, the film formation speed can bemade constant by regulating the crystal orientation, thereby obtaininguniform thickness of the formed dielectric film. Still further, thisreduces surface irregularities and thus leak current due to electricalfield concentration at irregularities.

With these effects, a dielectric film of high dielectric strength andreduced leak current is formed. Use of the dielectric film enables athin film capacitor.

DETAILED DESCRIPTION OF PREFERRED EXAMPLES EXAMPLE 1

An example of the production method of the capacitor according to thepresent invention is illustrated in FIGS. 7A to 7E.

<Step 1>

A silicon substrate (SiO₂ /Si) formed with a silicon oxide film 2 isprovided on which is formed a polycrystalline silicon film to athickness of about 3500 Å on the silicon oxide film 2 by a low pressureCVD method using a raw material gas (monosilane gas diluted with heliumto 50%) under conditions of a film formation temperature of 640° C., apressure of 16 Pa (monosilane partial pressure: 8 Pa), and a filmformation time of about 35 minutes (FIG. 7A).

<Step 2>

The substrate formed with the polycrystalline silicon film 3 is heattreated while forming an oxide film in a heat treatment furnace usingphosphorus oxychloride and oxygen as a source gas under conditions of950° C. for 10 minutes to form about 100 Å of a phosphosilicate glassfilm 6 on the polycrystalline silicon film 3. The phosphorus is doped inan amount of 1×10²⁰ atoms/cm³ to 1×10²¹ atoms/cm³. By this heattreatment, the crystal orientation of polycrystalline silicon isarranged in (111) (FIG. 7B)

<Step 3>

The phosphosilicate glass film 6 formed in step 2 is removed by etchingwith an HF solution (FIG. 7C).

<Step 4>

The substrate having the oriented polycrystalline silicon film 3a istreated in a heat treatment furnace at 1000° C. for 40 minutes usingnitrogen gas and oxygen gas to thermally oxidize the surface of theoriented polycrystalline silicon film, thus forming a dielectric film of30 nm.

<Step 5>

A polycrystalline silicon film is formed as in step 1 on the dielectricfilm as an upper electrode of the capacitor, which is treated as in step2 and step 3 to form an oriented polycrystalline silicon film 4a (FIG.7E).

<Step 6>

The oriented polycrystalline silicon film 4a is patterned byphotolithography using a mask to form the upper electrode and, further,a part of the oriented polycrystalline silicon film 3a as a lowerelectrode is exposed. Then, the upper electrode and the lower electrodeare wired by bonding gold wires or the like to obtain a capacitor (FIG.2).

EXAMPLE 2

A capacitor was produced using the same procedure as in the Example 1except that the monosilane partial pressure was set to 5 Pa. (TestExample 1)

Using the capacitors obtained by Examples 1 and 2 of the presentinvention, a capacitor as Comparative Example 1 produced using thepolycrystalline silicon film formed under the condition of a totalpressure of 22 Pa in step 1 of Example 1 and using the same subsequentprocedures as in Example 1, and a capacitor as Comparative Example 2produced using a phosphorus doping amount of 1×10¹⁹ atoms/cm³ in step 2of Example 1 and using the same subsequent procedures as in Example 1,were measured for dielectric strength.

As a result, the capacitors of Example 1 and Example 2 of the presentinvention showed a dielectric strength of no lower than 8 MV/cm, whereasthe capacitors of Comparative Example 1 and Comparative Example 2 bothhad a dielectric strength of no higher than 3 MV/cm.

Further, leak currents at an electrical field of 4 MV/cm were about 5pA/cm² for the capacitors of Examples 1 and 2 of the present invention,whereas about 5 nA/cm² for both capacitors of Comparative Examples 1 and2.

These results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                 Monosilane                                                                           Doping                                                                              Dielectric                                            Total pressure partial pressure amount strength Leak current                  (Pa) (Pa) (atoms/cm.sup.3) (MV/cm) (field: 4 MV/cm)                         __________________________________________________________________________    Example 1                                                                           16     8      1 × 10.sup.20 to                                                              no lower                                                                           about 5 pA/cm.sup.2                                 1 × 10.sup.21 than 8                                                 Example 2 10 5 1 × 10.sup.20 to no lower about 5 pA/cm.sup.2                                             1 × 10.sup.21 than 8                   Comp. Ex. 1 22 11  1 × 10.sup.20 to no higher about 5 nA/cm.sup.2          1 × 10.sup.21 than 3                                                 Comp. Ex. 2 16 8 1 × 10.sup.19 no higher about 5 nA/cm.sup.2                                              than 3                                    __________________________________________________________________________

As described above, enhancement of the dielectric strength of thedielectric film itself and reduction in the leak current, allowsreduction in the film thickness of the dielectric film, thereby reducingthe device size of the capacitor.

EXAMPLE 3

An example of the production method of the thin film transistoraccording to the present invention will be described with reference toFIGS. 8A to 8E.

<Step 1>

A silicon oxide (SiO₂) film 11 is formed on a silicon (Si) substrate 10,further on top of which is formed a polycrystalline silicon 12 to athickness of about 1500 Å by a low pressure CVD method using a materialgas (monosilane gas diluted with helium to 50%) at a film formationtemperature of 640° C. and a pressure of 10 Pa (monosilane partialpressure: 5 Pa) (FIG. 8A).

<Step 2>

The thus treated substrate is heat treated in a heat treatment furnacewhile forming an oxide film (phosphosilicate glass film) 16 usingphosphorus oxychloride and oxygen as a source gas at a temperature of950° C. By the heat treatment, the crystal orientation ofpolycrystalline silicon including the gate portion (body portion) isarranged in (111) (FIG. 8B).

<Step 3>

The phosphosilicate glass film 16 formed in step 2 is removed by etchingwith an HF solution (FIG. 8C).

<Step 4>

The substrate having the oriented polycrystalline silicon film 12a isthermally oxidized in a heat treatment furnace at 1000° C. to form adielectric film 13 of 1000 Å in thickness (FIG. 8D).

<Step 5>

An electroconductive polycrystalline silicon film 14a is formed on thedielectric film 13 (FIG. 8E).

<Step 6>

The polycrystalline silicon film 14a is patterned by photolithographyusing a mask to form a gate electrode and, further, the source and drainportions are doped to form electrodes. Then, the source electrode andthe drain electrode are wired by bonding gold wires 15 or the like toobtain a thin film transistor (FIG. 3).

By the method, in the thin film transistor, polysilicon is oriented in(111) by doping by heat treatment in a short time. As a result, atransistor having a high dielectric strength of the dielectric filmformed on the oriented polysilicon can be produced in a short time, eventhough the threshold voltage is increased because of an increasedimpurity concentration of the body portion (gate portion).

After the polycrystalline silicon is formed at a monosilane partialpressure of no more than 10 Pa in the material gas, the polycrystallinesilicon film is oriented in (111) to have a regular columnar structureby doping with the impurity in the heat treatment, and the dielectricfilm formed thereon has no steep portions, reducing generation ofelectric field concentration.

Further, by arranging the crystal orientation, the film formation speedis made constant, thereby forming a dielectric film of an uniformthickness.

INDUSTRIAL APPLICABILITY

With these effects, a dielectric film is formed which is high indielectric strength and small in leak current, thereby enabling a thinfilm structure of the capacitor.

Further, according to the present invention, a thin film transistor ofhigh dielectric strength can be produced in a short time.

What is claimed is:
 1. A semiconductor device comprising apolycrystalline silicon layer whose main crystal orientation is orientedin (111), an SiO₂ layer contacting said polycrystalline silicon layerobtained from said polycrystalline silicon layer, and an electrodecontacting said SiO₂ layer, wherein a height difference of surfaceirregularities of said polycrystalline silicon or said dielectric filmis no more than 30 nm.
 2. The semiconductor device as claimed in claim1, wherein said dielectric film has a dielectric strength of no lessthan 8 MV/cm.
 3. A capacitor having a dielectric film between a lowerelectrode and an upper electrode, wherein said lower electrode is apolycrystalline silicon layer which has a high conductivity and whosemain crystal orientation is oriented in (111), and said dielectric filmis an SiO₂ layer obtained from said polycrystalline silicon layer. 4.The capacitor as claimed in claim 3, wherein said polycrystallinesilicon layer contains an impurity in a concentration of 1×10²⁰atoms/cm³ to 1×10²¹ atoms/cm³.
 5. The capacitor as claimed in claim 4,wherein a height difference of surface irregularities of saidpolycrystalline silicon layer or said dielectric film is no more than 30nm.
 6. The capacitor as claimed in claim 5, wherein said dielectric filmhas a dielectric strength of no less than 8 MV/cm.
 7. The capacitor asclaimed in claim 4, wherein said dielectric film has a dielectricstrength of no less than 8 MV/cm.
 8. The capacitor as claimed in claim3, wherein a height difference of surface irregularities of saidpolycrystalline silicon layer or said dielectric film is no more than 30nm.
 9. The capacitor as claimed in claim 3, wherein said dielectric filmhas a dielectric strength of no less than 8 MV/cm.